The Central Dogma and Transcription

The Central Dogma

Overview of the Central Dogma

• The Central Dogma of molecular biology explains the flow of genetic information within a biological system.

Transcription

• Transcription is defined as the process through which DNA is used as a template to create messenger RNA (mRNA).
• Example nucleotide sequences:
  • DNA: A T G C C A T A C G G U
  • mRNA: (corresponding bases would be A U C G G U A U G C

Key Enzymes Involved in DNA Replication

• Helicase: Unzips the DNA double helix.
• Primase: Signals the starting point for replication.
• DNA Polymerase: Synthesizes new DNA strands.
• Exonuclease: Snips away RNA primers.
• Ligase: Glues together Okazaki fragments on the lagging strand.

RNA versus DNA

• RNA is single-stranded.
• Ribonucleotides contain ribose as their sugar.
• RNA includes the bases adenine, uracil, cytosine, and guanine.
• RNA can leave the nucleus, which is essential for its function in protein synthesis.

Transcription of mRNA

• In eukaryotes, transcription occurs in the nucleus.
• The enzyme responsible for transcription is RNA polymerase.
• RNA polymerase reads a gene on the DNA and synthesizes a complementary RNA strand.
• Definition of a gene: A gene is a DNA sequence that contains the instructions for making a specific protein.

Structure of a Gene

Each gene contains three main regions:

• Promoter Sequence: The binding site for RNA polymerase, crucial for the initiation of transcription.
• Protein-Encoding Region: The segment that is transcribed into mRNA and ultimately codes for a protein.
• Terminator Sequence: Signals the end of transcription, leading to the disassembly of the transcription machinery.

The Process of Transcription

Step 1: Initiation

• RNA polymerase binds to the promoter of a gene.
• It unwinds a small region of the DNA, breaking the hydrogen bonds to form a transcription bubble.

Step 2: Elongation

• RNA polymerase reads the template DNA strand and synthesizes a complementary RNA strand.
• RNA synthesis occurs in the 5’ to 3’ direction using base-pairing rules:
  • A pairs with U
  • T pairs with A
  • G pairs with C
  • C pairs with G

Step 3: Termination

• RNA polymerase reaches a termination site, causing it to become destabilized from the DNA template.
• The newly synthesized mRNA is then released.

mRNA Processing

• After mRNA is produced, it is initially known as pre-mRNA.
• Pre-mRNA requires additional processing before it can be translated at the ribosome.
• Types of mRNA Processing
  • Intron Splicing: The removal of introns from the mRNA sequence, which are non-coding regions.
  • 5’ Capping: The addition of a modified guanine nucleotide, called methylguanosine, to the 5’ phosphate end of the mRNA.
  • Addition of 3’ Tail: A stretch of adenines (approximately 200) is added to the 3’ end, known as a Poly-A tail, which helps stabilize the mRNA and aids in its export from the nucleus.

The Composition of mRNA

• Exons: Regions of the RNA that will be translated into protein.
• Introns: Non-coding regions of the RNA that are spliced out during mRNA processing.
• mRNA Splicing: The process where introns are removed, allowing the remaining exons to be joined together to form a functional mRNA molecule.

mRNA Modifications

• 5’ Cap: Enhances stability and protects the mRNA from degradation.
• 3’ Poly-A Tail: Provides protection from enzymatic degradation and aids in the transport of the mRNA to the ribosome for translation.

Comparison of DNA Replication and Transcription